Device and method for converting an ac voltage

ABSTRACT

The present invention relates to a device ( 10 ) for converting an AC voltage from the mains electricity supply into a DC voltage of predetermined level (and waveform), comprising:—a rectifier circuit ( 16, 17, 18, 19 ) for connecting to the mains electricity supply;—a switching circuit connected to the rectifier circuit;—a main transformer ( 26 ) connected to the switching circuit; and—an auxiliary transformer ( 28 ) which is connected to the switching circuit and the secondary winding of which is coupled to the secondary winding of the main transformer such that the current through the switching circuit and the main transformer is limited to a predetermined value.

For the purpose of charging laptops, computers, mobile phones and thelike there is a rapidly increasing demand for compact and low-pricedevices for converting a mains voltage to a desired DC voltage ofpredetermined value, for instance 12, 24 or 36 V, optionally withflattened waveform on the DC voltage side.

U.S. Pat. No. 3,660,724 discloses an improved protective circuit whereina silicon controlled rectifier is switched into circuit with a powersupply in the presence of excessive output voltage, so as to protect theelectronic equipment which is normally energized by the power supply.

DE-A-3427221 discloses a circuit in which the output voltage is comparedwith a reference voltage in a secondary circuit.

U.S. Pat. No. 4,272,805 discloses a switching regulator in which aninput DC voltage is chopped and then applied to drive a transformer fromwhich an AC voltage is derived which in turn is rectified in a rectifiercircuit.

Existing so-called converters of the linear type are large and heavy andconsume a considerable amount of power, even when no voltage is beingconsumed.

The present invention has for its object to obviate one or more of theabove stated problems.

The present invention provides a device for converting an AC voltagefrom the mains electricity supply into a DC voltage of predeterminedlevel (and waveform), comprising:

-   -   a rectifier circuit for connecting to the mains electricity        supply;    -   a switching circuit connected to the rectifier circuit;    -   a main transformer connected to the switching circuit; and    -   an auxiliary transformer which is connected to the switching        circuit and the secondary winding of which is coupled to the        secondary winding of the main transformer such that the current        through the switching circuit and the main transformer is        limited to a predetermined value.

As a result of the present invention a device for a mobile phone and thelike with a relatively small number of components can be obtained at arelatively low cost price.

In a preferred embodiment the mains voltage is filtered by means of arelatively small number of diodes of low cost price and a singlecapacitor and self-induction element, for instance in the so-calledButterworth configuration.

The switching device is preferably operated at a frequency of 64 kHz orhigher, so that humans and animals suffer little inconveniencetherefrom.

In a preferred embodiment use is made of a so-called Common-Base powertransistor, which in further preferred embodiments is protected againsttoo high a current and too high a temperature.

This protection is preferably achieved by means of a diode on thesecondary side of the auxiliary transformer, although a (Zener) diode ofrelatively low cost price on the primary winding of the auxiliarytransformer can likewise be envisaged in order to hold the temperatureof the power transistor at about 80° C. to 85° C., in any event clearlybelow 100° C. The limitation in the temperature rise of this powertransistor depends partly on the steepness of the characteristic of the(Zener) diode on the primary or secondary side of the transformer, or onthe sharp bend in the characteristic thereof, so as to limit currentvalue.

The transistor is preferably a bipolar transistor, the cost price ofwhich is a factor of about 2.5 lower than a usual power transistor ofthe MOSFET type.

The present invention requires relatively few components of standardmake, so that the cost price can remain low.

Further advantages, features and details of the present invention willbe elucidated on the basis of the following description of a preferredembodiment thereof, with reference to the annexed figure showing acircuit diagram thereof.

A device 10 is provided on one side with connecting terminals 11, 12 forconnecting to the mains supply, and on the other side with connections13 and 14 to which can be connected a battery of a mobile phone, laptopand the like. Device 10 is provided with a fuse 15 and four diodes 16,17, 18, 19 for rectifying an AC voltage of for instance 120 V to aso-called rail or bus voltage of about 170 V. Circuit 10 furthercomprises two capacitors 21 and 22 and, connected therebetween, aself-induction element 23 in so-called Butterworth configuration for thepurpose of filtering the rectified voltage. Further connected to line 25is the primary winding of a transformer 26 which is connected on theother side to a power transistor 27 which is connected in the so-calledCommon Base configuration and the emitter of which is connected to theprimary winding of an auxiliary transformer 28, which is connected onthe other side via resistor 29 and a temperature-sensitive NTC resistor30 to the base of transistor 27. Between the base of power transistor 27and line 25 are further connected a diode 31 and a self-inductionelement 32. Further connected to the base of power transistor 27 is aDIAC 33 with thyristor action, which is connected in turn to line 25 viaa resistor 34. The node 35 between DIAC 33 and resistor 34 is furtherconnected via a capacitor 36 of relatively small capacity to the emitterwith power transistor 27 and further via a diode 37 to the collectorthereof. The secondary winding of auxiliary transistor 28 is connectedto the secondary winding of main transistor 26 via a diode 38. A seconddiode 39 is connected in series with diode 38 to the secondary windingof transistor 26, while in the shown preferred embodiment two capacitors40, 41 are connected. Via a capacitor 42 the connecting terminal 14 isconnected to the base of power transistor 27.

After energizing of connecting terminals 11 and 12, for instance up toan AC voltage of 120 V, capacitors 21 and 22 can be charged up to avoltage of for instance 170 V. Capacitor 36 will be charged via resistor34 until DIAC 33 comes into operation, for instance at a voltage of 45V. Capacitor C3 will now be discharged to the emitter of transistor 27,whereby a voltage will appear at the primary winding of main transformer26. The output capacitors 40 and 41 will be immediately charged, whilethe current with which these capacitors are charged can be fed back tothe emitter of transistor 27, whereby this latter remains in conductivestate.

Transistor 27 continues to conduct as long as the emitter current isgreater than the base current. As soon as main transformer 26 orauxiliary transformer 28 becomes saturated or current begins to flowthrough diode 38, the transistor will be switched off.

At low load the transistor will be switched off due to saturation ofmain transformer 26. At higher load the voltage on the primary windingof auxiliary transformer 28 will increase, wherein transistor 27 must beswitched off when auxiliary transformer 28 is saturated. Powertransistor 27 will also switch off when the diode 38 begins to conduct,which means that a current is flowing through resistors 29 and 30. Powertransistor 27 will be switched off when the closed loop amplificationhas decreased sufficiently. As transistor 27 heats up the resistancevalue of resistor 30 will increase, whereby power transistor 27 willeventually be switched off.

Restarting of transistor 27 (Q2) takes place automatically usingself-induction 32.

As soon as a relatively small current begins to flow in a secondarywinding of main transformer 26, a correspondingly small current in theemitter of power transistor 27 will soon switch the latter on again.

The above described preferred embodiment of a device according to thepresent invention with an output of 15-20 Watt can be arranged in thehousing of a standard M5 type. The weight can amount to less than 100grams, while the device will consume less than 0.1 Watt of power if noload is connected to terminals 13, 14, while the output voltage ismaintained.

With a relatively small number of components, i.e. auxiliary transformer28, diode 38 and the temperature-sensitive resistor 30, the abovedescribed preferred embodiment of the device according to the presentinvention will achieve protection of the converter against too great anoutput as well as regulation of the power thereof. It will be possiblein many cases to omit the temperature-sensitive resistor 30, dependingfor instance on the protective devices already available on connectingterminals 13 and 14.

The present invention is not however limited to the above describedpreferred embodiment thereof; the rights sought are defined by thefollowing claims, within the scope of which many modifications can beenvisaged.

1. Device for converting an AC voltage from the mains electricity supplyinto a DC voltage of predetermined level (and waveform), comprising: arectifier circuit for connecting to the mains electricity supply; aswitching circuit connected to the rectifier circuit; a main transformerconnected to the switching circuit; and an auxiliary transformer whichis connected to the switching circuit and the secondary winding of whichis coupled to the secondary winding of the main transformer such thatthe current through the switching circuit and the main transformer islimited to a predetermined value.
 2. Device as claimed in claim 1,wherein a filter circuit is connected between the rectifier circuit andthe switching circuit.
 3. Device as claimed in claim 2, wherein thefilter circuit comprises a number of diodes. 4-12. (canceled)
 13. Deviceas claimed in claim 2, wherein the filter circuit comprises at least onecapacitor and one self-induction element.
 14. Device as claimed in claim3, wherein the filter circuit comprises at least one capacitor and oneself-induction element.
 15. Device as claimed in claim 1, wherein theswitching circuit comprises a power transistor in common-baseconfiguration.
 16. Device as claimed in claim 2, wherein the switchingcircuit comprises a power transistor in common-base configuration. 17.Device as claimed in claim 3, wherein the switching circuit comprises apower transistor in common-base configuration.
 18. Device as claimed inclaim 13, wherein the switching circuit comprises a power transistor incommon-base configuration.
 19. Device as claimed in claim 14, whereinthe switching circuit comprises a power transistor in common-baseconfiguration.
 20. Device as claimed in claim 15, wherein the switchingcircuit comprises a DIAC.
 21. Device as claimed in claim 16, wherein theswitching circuit comprises a DIAC.
 22. Device as claimed in claim 17,wherein the switching circuit comprises a DIAC.
 23. Device as claimed inclaim 18, wherein the switching circuit comprises a DIAC.
 24. Device asclaimed in claim 19, wherein the switching circuit comprises a DIAC. 25.Device as claimed in claim 15, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 26.Device as claimed in claim 16, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 27.Device as claimed in claim 17, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 28.Device as claimed in claim 18, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 29.Device as claimed in claim 19, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 30.Device as claimed in claim 20, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 31.Device as claimed in claim 21, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 32.Device as claimed in claim 22, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 33.Device as claimed in claim 23, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 34.Device as claimed in claim 24, wherein the auxiliary transformer isconnected to the collector or emitter of the power transistor. 35.Device as claimed in claim 1, wherein a diode is connected between theconnecting terminals of the secondary windings of the auxiliarytransistor.
 36. Device as claimed in claim 30, wherein a diode isconnected between the connecting terminals of the secondary windings ofthe auxiliary transistor.
 37. Device as claimed in claim 31, wherein adiode is connected between the connecting terminals of the secondarywindings of the auxiliary transistor.
 38. Device as claimed in claim 32,wherein a diode is connected between the connecting terminals of thesecondary windings of the auxiliary transistor.
 39. Device as claimed inclaim 33, wherein a diode is connected between the connecting terminalsof the secondary windings of the auxiliary transistor.
 40. Device asclaimed in claim 34, wherein a diode is connected between the connectingterminals of the secondary windings of the auxiliary transistor. 41.Device as claimed in claim 36, wherein a resistor is connected betweenthe base and the collector or emitter of the power transistor. 42.Device as claimed in claim 37, wherein a resistor is connected betweenthe base and the collector or emitter of the power transistor. 43.Device as claimed in claim 38, wherein a resistor is connected betweenthe base and the collector or emitter of the power transistor. 44.Device as claimed in claim 39, wherein a resistor is connected betweenthe base and the collector or emitter of the power transistor. 45.Device as claimed in claim 40, wherein a resistor is connected betweenthe base and the collector or emitter of the power transistor. 46.Device as claimed in claim 41, wherein a resistor withtemperature-dependent value is connected between the base of the powertransistor on the one hand and the collector or emitter on the other.47. Device as claimed in claim 42, wherein a resistor withtemperature-dependent value is connected between the base of the powertransistor on the one hand and the collector or emitter on the other.48. Device as claimed in claim 43, wherein a resistor withtemperature-dependent value is connected between the base of the powertransistor on the one hand and the collector or emitter on the other.49. Device as claimed in claim 44, wherein a resistor withtemperature-dependent value is connected between the base of the powertransistor on the one hand and the collector or emitter on the other.50. Device as claimed in claim 45, wherein a resistor withtemperature-dependent value is connected between the base of the powertransistor on the one hand and the collector or emitter on the other.